Hydrostatic transmissions having a pair of hydraulic displacement units connected in a closed loop circuit are well known. One of the displacement units functions as a pump and the other unit functions as a motor. The pump is normally variable and operates to supply fluid under pressure in a high pressure line side of the closed loop circuit to the motor. A low pressure line of the closed loop circuit directs fluid from the motor back to the pump. Various controls for protecting the hydrostatic transmission from over-pressure are conventional, and include pressure relief valves, pressure compensators, and pressure limiters.
U.S. Pat. No. 4,617,797 describes a pair of multi-function valves for a hydrostatic transmission. The multi-function valves each provide a pressure limiter function, a high pressure relief function, a charge check valve function, and a bypass function. A displacement control valve modulates the hydraulic pressure in a pair of servo cylinders such that the proper unit output is maintained. However, if the system pressure becomes higher than a pre-set amount, the multi-function valve ports oil into the lower servo which is tanked across a built-in orifice in the displacement control valve. The flow to tank across the orifice generates a back pressure in the servo, allowing the swashplate to readjust to a lower pressure. This flow is also used as the pilot flow for the high pressure relief valve function of the multi-function valve.
Among the problems of multi-function valves are that loss of flow can adversely affect other functions; viscosity changes sometimes result from increased temperature which may affect fluid leakage; high and low charge pressures can adversely affect the pressure limiter; the large size of the orifice in the high pressure relief valve sometimes inhibits the valve from fully opening; the endcaps must be configured either as lefthand or righthand rotation assemblies which is expensive; the housing castings are more complex and difficult to create; and contamination entering the clearance between the piston and the bore is seldom flushed out.
Accordingly, a primary objective of the present invention is the provision of an improved hydrostatic transmission.
Another objective of the present invention is the provision of a pilot-operated pressure override valve for a hydrostatic transmission which overcomes the problems associated with circuitry using a direct connection between the multi-function valves and the displacement control valve.
A further objective of the present invention is the provision of a hydrostatic transmission having a selective configuration for directing oil from either or both of the multi-function valves to or around a pressure override valve.
Another objective of the present invention is the provision of a pilot-operated pressure override valve having flow through the valve.
Still another objective of the present invention is the provision of a pressure override valve which is not sensitive to the rotational direction of the pump shaft in a hydrostatic transmission.
A further objective of the present invention is the provision of a pilot-operated pressure override valve which maintains a substantially constant delta pressure.
Yet another objective of the present invention is the provision of a pilot-operated pressure override valve having minimal variability of set pressure.
Another objective of the present invention is the provision of a pilot-operated pressure override valve having no loss flow from the charge circuit.
Still another objective of the present invention is the provision of a non-dissipative pilot-operated pressure override valve.
These and other objectives will become apparent from the following description of the invention.